In wellbore exploration, typically drilling mud such as oil-based mud and synthetic-based mud are used. Filtrates from these muds generally invade the hydrocarbon bearing formation through the borehole wall. Thus, samples taken from the formation contain drilling mud filtrate contamination. Thus, a sufficient volume of fluid must pumped from the formation to reduce the filtrate contamination in the sample to an acceptable level. Open-hole sampling is an effective way to acquire representative formation fluid samples. Formation fluid sample acquisition and analysis allows determination of critical information for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. In openhole sampling, initially, the flow from the formation contains a considerable quantity of filtrate, but as filtrate is drained from the formation, the flow increasingly becomes richer in formation fluid and less filtrate appears in the flow. That is, the composition of fluid flowing from the formation progresses towards a higher percentage of native formation fluid but a lower percentage of filtrate as pumping continues and the filtrate that had invaded the formation is depleted.
Thus, fluid being pumped from a wellbore undergoes a clean-up process in which the purity of the sample increases over time as filtrate is gradually removed from the formation so that less filtrate appears in the sample. As the composition of the sampled formation fluid changes, so do the optical and physical properties of the sampled fluid, such as optical absorption, fluorescence, refractive index, density, and viscosity. A number of different measurements are used to determine various optical and physical properties of a fluid downhole in real time. Measuring these properties of the fluid therefore provides insight into a sample's purity.
When extracting fluids from a formation, it is desirable to quantify the cleanup progress, that is, the degree contamination from filtrate in the formation fluid sample in real time. If it is known that there is too much filtrate contamination in the sample (e.g., more than about 10% filtrate), then there is little reason to collect a formation fluid sample in a sample tank. One should wait until the contamination level drops to an acceptable level. On the other hand, if by pumping for a relatively long time, it is possible to achieve an only slightly better filtrate contamination level, an operator may end up wasting very expensive rig time and also risks the very costly possibility of allowing a tool to become stuck in the wellbore.
When pumping first begins, the fluid being pumped contains a large amount of mud filtrate contamination but the fluid filtrate percentage is decreasing at the fastest rate. This process of decreasing fluid filtrate contamination is referred to as sample clean up. Later, the pumped fluid contains less contamination but the fluid filtrate percentage decreases at a slower rate. One way to monitor cleanup is to monitor the increase in gas oil ratio (GOR) as pumping continues and the flow from the formation cleans up from mostly gas-free oil-based mud filtrate and to mostly gas-containing oil. Oil companies are also very interested in knowing the GOR of the crude oils that they find downhole independent of using GOR as a cleanup monitor. Thus, there is a need for a method and apparatus for determining GOR in real time downhole.